The present invention relates to a burnishing operation and more particularly to a burnishing tool which provides a cold-working operation on a non-linear profile and to the method of burnishing a workpiece such as an aircraft wheel.
Burnishing is often used as a finishing operation after a workpiece has been machined or ground to eliminate minute surface irregularities. Burnishing is to be distinguished from a spinning operation wherein the tool is urged against the workpiece blank which is deformed into a desired shape usually in a series of passes with substantial deformation of the material. These operations are distinct and operate under different principles. A burnishing operation as herein used is a cold-working operation which does not remove material but compresses the microscopic peaks of a metal surface into adjacent valleys as if to densify the surface thereby obtaining a superior finish that has improved physical properties. Herein the burnishing condenses the grain structure of the metal producing a superior hard, corrosion resistant long-wearing surface, free of grit, and the microscopic peaks and valleys that are inherent in metal turning operations. In many instances, the burnishing tool operates on a linear surface. An example of this is a burnishing tool composed of a series of tapered, highly polished and hardened rolls journaled in circumferentially spaced recesses of a carrier member. Each roll has its axis parallel to the axis of the carrier member and is used to burnish the inside diameter surface of a tubular product. In these instances the roller burnishing tool can be used to help achieve size control while producing a superior finish. The burnishing tool holder of the present invention is directed to the use of a narrow radius roller that not only can burnish a linear outside diameter surface but can also finish any outside diameter (O.D.) surface configuration containing fillets, radii, tapers and grooves. The burnishing tool of the present invention is mounted on a tool holder supported by a slide or turret support that is moveable in two perpendicular directions which can be dictated by a numerically controlled input means. The turret support is generally moved in its first or longitudinal direction by a feed screw that is parallel to the axis of the workpiece with the second direction being transverse thereto and for illustration purposes is numerically controlled. Such a numerically controlled path dictates the transverse movement so that the burnishing tool can effect a controlled pressure on the workpiece to effect the cold working processing. One of the most effective means for controlling such burnishing operations heretofore has been to use a pneumatic cylinder device regulated by a control valve to control the transverse movement and constant pressure. Such control valve device has the drawback in that the thrust of the burnishing tool is not sensitive enough to properly take into account the frictional forces and the rapidly changing contour of a fillet as on a wheel rim to maintain the proper pressure or when one changes from an axial movement to a compound transverse and axial movement, particularly on thin workpieces or workpieces of differential thickness. The burnishing tool of the present invention utilizes a generally U-shaped support whose bite portion has a circumferentially extending loop portion which can take into account the frictional forces and is highly responsive and sensitive to contour changes to enable the proper cold working of such surfaces. The applied pressure can be reduced when a thin wall section is encountered so that workpiece is not distorted. The present invention provides the unique feature that the compound movement of the slides that support the resilient burnishing tool can be controlled by either a cam as a template or numerically controlled which positions these slides at precise locations in their compound movement so that the resilient burnishing tool holder and burnishing tool can provide a variable force as it works on a thin walled section of a workpiece followed by a thick walled section along the same horizontal line that is parallel to the axis of rotation of the workpiece. Heretofore the regulating valve could not discern a thin walled section and would severly alter its contour or cause an undesirable flow of metal.